Fig. 1. Mechanisms of neutralization of an enveloped virus by neutralizing antibodies in vitro.

Antibody-mediated neutralization of enveloped viruses in vitro can operate by several mechanisms. a, Preventing the attachment of virions to host cell receptors by the disassembly or conformational modification of viral spike proteins. b, Aggregation of virions, impeding attachment to host cell receptors. c, Directly blocking binding of viral spike protein to host cell receptors through steric obstruction. d, Blocking the fusion of viral and host cell membranes by steric obstruction. e, Blocking conformational changes in spike protein required for virus entry into host cells. f, For viruses that enter endosomes, blocking entry into the cytoplasm by blocking endosomal cleavage and/or endosomal receptor binding. g, Blocking viral egress from the cell; although this is not strictly a mechanism of neutralization, it would be observed in multiple-round, but not single-round, neutralization assays and can occur through the aggregation of progeny virions at the surface of infected cells. The figure illustrates neutralization of an enveloped virus. For non-enveloped viruses, there is additionally intracytoplasmic neutralization involving the ubiquitin ligase TRIM21, which targets antibody-bound virus to the proteasome for degradation. Finally, the mechanisms that prevent viruses gaining entry to host cells can also prevent viruses spreading from one cell to another directly by inhibiting attachment of the infected cell to the uninfected cell and/or by inhibiting fusion of the membranes of the two infected cells.